A cup of coffee cools from 90 to 80 in t minutes when the room temperature is 20 . The time taken by the similar cup of coffee to cool from 80 to 60 at the same room temperature is:
(1) 135t
Newton's law of cooling (average form): T t=k (Tavg-T0 )
Case 1: 10t=k(85-20)=65k
Case 2: 20t'=k(70-20)=50k
Divide: t'2t= 6550
∴ t'= 135t
Answer: (1)
The temperature of a body in air falls from 40 to 24 in 4 minutes. The temperature of the air is 16 . The temperature of the body in the next 4 minutes will be :
Which of the following figure represents the relation between Celsius and Fahrenheit temperatures?
A gun fires a lead bullet of temperature 300 K into a wooden block. The bullet having melting temperature of 600 K penetrates into the block and melts down. If the total heat required for the process is 625 J, then the mass of the bullet is grams. (Latent heat of fusion of lead =2.5 104 J Kg-1 and specific heat capacity of lead =125 J Kg-1 K-1)
Two spherical bodies of same materials having radii 0.2 m and 0.8 m are placed in same atmosphere. The temperature of the smaller body is 800 K and temperature of the bigger body is 400 K . If the energy radiated from the smaller body is E, the energy radiated from the bigger body is (assume, effect of the surrounding temperature to be negligible),
An amount of ice of mass 10-3 kg and temperature -10 C is transformed to vapour of temperature 110 C by applying heat. The total amount of work required for this conversion is, (Take, specific heat of ice =2100 J kg-1 K-1, specific heat of water =4180 J kg-1 K-1, specific heat of steam =1920 J kg-1 K-1, Latent heat of ice =3.35 105 J kg-1 and Latent heat of steam =2.25 106 J kg-1 )
Three conductors of same length having thermal conductivity k1,k2 and k3 are connected as shown in figure. [figure] Area of cross sections of 1st and 2nd conductor are same and for 3rd conductor it is double of the 1st conductor. The temperatures are given in the figure. In steady state condition, the value of is . (Given : k1=60 Js-1 m-1 K-1, k2=120 Js-1 m-1 K-1, k3=135 Js-1 m-1 K-1 )